Allied Telesis AT-S82 - Manual (Page 64)

Chapter 8: Spanning Tree Protocol (STP)
64
Path cost is determined through an evaluation of port costs. Every port on
a bridge participating in STP has a cost associated with it. The cost of a
port on a bridge is typically based on port speed. The faster the port, the
lower the port cost. The exception to this is the ports on the root bridge,
where all ports have a port cost of 0.
Path cost is simply the sum of the port costs between a bridge and the root
bridge.
The AT-GS950/8 WebSmart switch automatically sets the port cost
according to the speed of the port, assigning a lower value for higher
speeds. Table 6 lists the STP port costs.
Table 5 lists the STP port costs with Auto when a port is part of a port
trunk.
Table 6 lists the RSTP port costs with Auto.
Table 4. STP Port Costs
Port Speed Port Cost
10 Mbps 100
100 Mbps 10
1000 Mbps 4
Table 5. STP Auto-Detect Port Trunk Costs
Port Speed Port Cost
10 Mbps 4
100 Mbps 4
1000 Mbps 2
Table 6. RSTP Auto Port Costs
Port Speed Port Cost
10 Mbps 2,000,000
100 Mbps 200,000
1000 Mbps 20,000